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PHYSICS, M6 2019 VCE 3

Figure 3 shows a schematic diagram of a DC motor. The motor has a coil, \(JKLM\), consisting of 100 turns. The permanent magnets provide a uniform magnetic field of 0.45 T.

The commutator connectors, \(X\) and \(Y\), provide a constant DC current, \(I\), to the coil. The length of the side \(JK\) is 5.0 cm.

The current \(I\) flows in the direction shown in the diagram.
 

  1. Which terminal of the commutator is connected to the positive terminal of the current supply?   (1 mark)

  2. Draw an arrow on Figure 3 to indicate the direction of the magnetic force acting on the side \(JK\).   (1 mark)

  3. Explain the role of the commutator in the operation of the DC motor.   (2 marks)

  4. A current of 6.0 A flows through the 100 turns of the coil \(JKLM\).
  5. The side \(JK\) is 5.0 cm in length.
  6. Calculate the size of the magnetic force on the side \(JK\) in the orientation shown in Figure 3. Show your working.   (2 marks)

Show Answers Only

a.    → Current runs from the positive terminal to the negative terminal.

→ \(X\) is connected to the positive terminal of the current supply.
 

b.    Using the right-hand rule: force on \(JK\) is down the page.

c.    Role of the split-ring commutator:

→ Changes the direction of the current through the arms of the coil every 180\(^{\circ}\).

→ This ensures that the torque through the motor will always be in the same direction so the motor will rotate in the same direction.
 

d.    \(F=nlIB=100 \times 0.05 \times 6 \times 0.45 = 13.5\ \text{N}\)

Show Worked Solution

a.    → Current runs from the positive terminal to the negative terminal.

→ \(X\) is connected to the positive terminal of the current supply.
 

b.    Using the right-hand rule: force on \(JK\) is down the page.

c.    Role of the split-ring commutator:

→ Changes the direction of the current through the arms of the coil every 180\(^{\circ}\).

→ This ensures that the torque through the motor will always be in the same direction so the motor will rotate in the same direction.
 

d.    \(F=nlIB=100 \times 0.05 \times 6 \times 0.45 = 13.5\ \text{N}\)

PHYSICS, M6 2021 VCE 5 MC

The diagram below shows a small DC electric motor, powered by a battery that is connected via a split-ring commutator. The rectangular coil has sides KJ and LM. The magnetic field between the poles of the magnet is uniform and constant.
 

The switch is now closed, and the coil is stationary and in the position shown in the diagram.

Which one of the following statements best describes the motion of the coil when the switch is closed?

  1. The coil will remain stationary.
  2. The coil will rotate in direction A, as shown in the diagram.
  3. The coil will rotate in direction B, as shown in the diagram.
  4. The coil will oscillate regularly between directions A and B, as shown in the diagram.
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\(C\)

Show Worked Solution

→ When the switch is closed, the current through the side \(JK\) will run from \(J\) to \(K\) and the direction magnetic field is to the right.

→ Using the Right hand rule, the force on side JK will be down and the coil will rotate in direction B (anti-clockwise).

\(\Rightarrow C\)

♦ Mean mark 51%.

PHYSICS, M6 2023 HSC 25a

The diagram represents one type of electric motor.
 

Describe the function of part \( X \).  (2 marks)

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→ Part \(X\) is a split ring commutator and its function is to reverse the direction of the current through the arms of the armature.

→ This ensures that unidirectional torque is maintained in the DC motor so that it continues to rotate in the same direction. 

Show Worked Solution

→ Part \(X\) is a split ring commutator and its function is to reverse the direction of the current through the arms of the armature.

→ This ensures that unidirectional torque is maintained in the DC motor so that it continues to rotate in the same direction. 

PHYSICS, M6 2016 HSC 20 MC

In the motor shown, the rotor spins clockwise, as viewed from point `P`, when connected to a DC supply.
 

What happens when the motor is connected to an AC supply?

  1. There is no movement of the rotor.
  2. The rotor produces clockwise movement only.
  3. The rotor vibrates at the frequency of the AC supply.
  4. The rotor continuously turns half a rotation clockwise, then half a rotation anticlockwise.
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`B`

Show Worked Solution

→ Initially, the electromagnet on the left has a magnetic south pole on its side closest to the coil, the electromagnet on the right has a north pole on its side closest to the coil and the current through the coil is anticlockwise as viewed from above.

→ Using the right hand palm rule, the rotor will rotate clockwise.

→ When the direction of AC current changes, the direction of current through the coil swaps, and the polarity of the electromagnets swaps.

→ This means that the direction of torque is maintained and the rotor continues to rotate clockwise.

→ The split-ring commutator functions as normal to maintain the direction of torque once the rotor has rotated through half a turn preventing the rotor from continuously turning half a rotation clockwise, then half a rotation anticlockwise.

→ The rotor produces clockwise movement only.

`=>B`


♦♦♦ Mean mark 17%.

PHYSICS, M6 2018 HSC 10 MC

The diagram shows some parts of a simple DC motor.
 

Which row of the table correctly describes the direction of force acting on side `WX` and the direction of torque this produces on the coil?
 

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`C`

Show Worked Solution

→ To ensure the motor rotates continuously in one direction, the direction of torque must remain constant.

→ So, the split ring commutator switches the direction of the current through the coil every 180°, reversing the direction of the force on `WX` every 180°.

`=>C`


Mean mark 52%.